The mammalian suprachiasmatic nucleus (SCN) has been implicated as a primary component in central nervous system mechanisms governing circadian rhythms. Disruption of the normal synchronization of temperature, activity, and other rhythms has an enormous effect on health. Sleep-wake disorders, decreases in vigilance and performance, and certain affective disorders may result from or be exacerbated by desynchronization of coupled rhythmic events. In order to understand how circadian rhythms are entrained by exogenous or endogenous factors, it is necessary to understand the neural mechanisms of both generation of rhythmic activity (pacemaker functions) and synchronization of internally generated rhythms by the environment (entrainment). Previous studies on the neurophysiology of the SCN have utilized the in vitro slice technique or single-unit recording from anesthetized animals. However, problems exist interpreting results from these types of preparations and further work must focus on recordings of cells in unanesthetized, unrestrained animals with afferent input intact. Parylene-coated, etched floating microelectrode bundles were developed for extracellular recording from the small somata in the preoptic and anterior hypothalamic nuclei and will now be used here for recordings from single SCN neurons in behaving animals. The aims of the proposal are to: 1) Investigate the changes in SCN unit activity as a function of time of day and sleep/wakefulness transitions; 2) Determine short and long-term changes in the firing rates and patterns of SCN cells as a function of photic stimulation; 3) Describe the thermosensitivity of single SCN neurons to again insight into the temperature-dependence of rodent rhythm generating mechanisms; 4) Study the effects of pentobarbital and urethane anesthesia on SCN neuronal characteristics; and 5) Relate structural specificity to function by electricl stimulation of afferent pathways and categorization of SCN cell type positions within the SCN.